A conventional machining center disclosed in European Patent No. EP1747843B2 includes a machine bed, a worktable, a slide assembly, a lifting device, a bar guide, a mounting element and a pallet holder. The machine bed has top and bottom surfaces that are opposite to each other. A machining zone and a rear loading zone are defined between the top and bottom surfaces and are arranged along a horizontal direction. The worktable is disposed in the machining zone and is rotatably mounted to the machine bed. The slide assembly is movably mounted to the top surface of the machine bed and is slidable along a first axis that extends in the horizontal direction. The lifting device is disposed in the rear loading zone, is mounted to the machine bed, and telescopically extends along a vertical axis. The bar guide is mounted to the lifting device and extends in the horizontal direction. The mounting element is movably mounted to the bar guide, and is slidable along a second axis that extends in the horizontal direction. The pallet holder is rotatably mounted to the mounting element, and has two holding portions that are disposed at opposite ends thereof.
Prior to a machining process, a workpiece pallet is mounted to one of the holding portions of the pallet holder distal from the machining zone, and the lifting device is operated to move the pallet holder upwardly, thereby raising the workpiece pallet. Next, the mounting element is operated to rotate the pallet holder about the vertical axis by 180 degrees, such that the holding portion carrying the workpiece pallet is now proximate to the machining zone. Then, the mounting element slides along the second axis relative to the bar guide so as to transfer the workpiece pallet from the rear loading zone into the machining zone. After that, the workpiece pallet is lowered by the lifting device and is disposed on the worktable, so that the slide assembly may be operated to actuate the machining process. After the machining process is finished, the workpiece pallet is transferred back to the rear loading zone to be prepared for the next machining process. More specifically, another workpiece pallet is mounted to the other one of the holding portions of the pallet holder so that when the pallet holder is rotated by 180 degrees again, the two workpiece pallets may switch places with each other.
Due to the abovementioned exchanging process of the workpiece pallets, such machining center is designed to accommodate rotation of the pallet holder which may easily take up sizeable amount of space.